There's a contradiction here. If your MBTI type isn't 'fixed' and is based on how you decide to answer questions, then it implies that you can choose your own outcome and MBTI type, rendering it meaningless.
If your MBTI is fixed because it is based on how you answer questions so long as it is without the intent to alter its outcome, then it is not anymore deterministic than a horoscope.
At the end of the day, people see what they want to in themselves and discard the rest, whether it's reading a horoscope or MBTI profile.